I actually found a much simpler method of installing osu! to ubuntu 13.04. Someone may already have found it, but I am posting the topic just in case.
You will need to do all the steps involving installing wine, and .net, yes. But the easiest part, you download the installer that you would normally use for the MAC, since it is a ZIP. You extract the ZIP file, and you have the entire osu! folder where you extracted it. All the files are there, including the game exe, and osume.exe. After that, simply use wine to run osume.exe, which will put the rest of the files into your folder, and then you can run the game exe from there. Simple and fast.
You will need to do all the steps involving installing wine, and .net, yes. But the easiest part, you download the installer that you would normally use for the MAC, since it is a ZIP. You extract the ZIP file, and you have the entire osu! folder where you extracted it. All the files are there, including the game exe, and osume.exe. After that, simply use wine to run osume.exe, which will put the rest of the files into your folder, and then you can run the game exe from there. Simple and fast.